Digester



OCI. 1l, 1955 J, D WALKER 2,720,489

DIGESTER Filed NGV. 22, 1950 bwm@ United States Patent rOttico 2,720,489Patented Oct. 11, 1955 DIGESTER James Donald Walker, Aurora, Ill.,assignor to Walker Process Equipment Inc., Aurora, Ill., a corporationof Illinois j Application November 22, `15150, Serial No. 197,047

7 Claims. (Cl. 210-2) In the digestion of sludge from sewage treatmentunits and the like, it has long been customary to carry the sludge toadigester, a large tank in which it is held for relativelylong periodsof time. In these digester tanks, the solids are digested or stabilizedin the course of time by anaerobic bacteria. The digesting process isaccelerated by maintaining an optimum temperature. This requires theapplication of considerable heat during cold weather.

Until recently, the most common method of heating digesters involved theuse of coils for circulating hot water through the tank,usually justinside the walls thereof. This involved a relatively expensiveinstallation and could cause considerable trouble if not properlyregulated at all times. Excessive temperatures could cause encrustationon the pipes, reducing their etiiciency. Cleaning olf such encnlstationswas extremely obnoxious and even dangerous.`

According to another form of heating, sludge from the digester contentswas drawn oft", passed through a heat exchanger where such contents wereraised to the desired temperature, and then returned to the tank. Thehigher speed of movement of the slurry through the heat exchanger pipesreduced the likelihood of` ,encrustation, thereby permitting moretolerance in the controls. Furthermore, the heat exchangers could be`designed to facilitate cleaning of the encrustation, should it occur,the flow of obnoxious gases meanwhile being cut off so that there was nodanger or annoyance from them.

`It has long been recognized that there were some desirable aspects inhaving a constant movement or circulation within the digester tank. Ithas been thought that the circulation of slurry from the digester andback would be advantageous in this regard. It hasalso been thought thatconvection currents from the heating coils within the digester mightproduce beneficial results. Neither method of circulation has provedvery satisfactory,` Not only have they failed to produce the mostdesirable movements within the tank but they have also made moredifficult the withdrawal of clear eflluent. It should be understood thatto carry the sludge or other solids into the tank, much more water wasrequired than could remain within the tank. It has been the practice tohave draw-oit pipes at several levels and to try to select a level atwhich relatively clear liquid could be drawn off. With circulation, ithas been dicult to nd such a level. In any event, changing conditionsare likely to keep any level from being dependable.

According to the present invention, the effluent is drawn olf` through aclarier which ensures a relatively clear efliuent. Considered alone,this is the subject of copending application Serial No. 103,235, liledJuly 6, 1949, abandoned after this application was led. This claricationof eilluent makes tolerable a much more pronounced circulation withinthe` tank than might otherwise be considered acceptable. This newtolerance is made use of by heating the contents of the tank by directinjection of hot gases in a way causing eitective circulation. For

thermal eiiiciency, the hot gases must be liberated well below thesurface of the liquid so that most of their heat will be given olf tothe liquid. This produces a very pronounced circulation throughout theupper portion of the digester tank, the upwardly moving gases acting onthe airlift principle to raise large bodies of the liquor in thatvicinity.` This heated and gasied liquor tends to stay on top of thecolder liquor in the tank and hence it spreads out to the far walls ofthe tank, displacing colder liquor and by its momentum helps set up abroad annular roll. On this is superimposed a downward drift of thecooler liquid and of the solids.

This liberation of heat by direct injection of hot gases may beaccomplished very eiiciently. Digesters produce a burnable gas. This gasmay be drawn oft", pumped to a pressure slightly in excess of thehydrostatic pressure at the point of ultimate liberation, burned withair pumped to a like pressure, and passed to the gas liberator. Theproducts of this combustion, if conditions are eticient, will be largelyCO2. To avoid excessive dilution of the combustible gases with suchspent products of combustion, they are drawn oi separately by means of asmall hood over the area of liberation. Being substantially free fromthe obnoxious unburned gases formed in the digester, these products ofcombustion may pass to the atmosphere from the hood.

Additional objects and advantages of the invention will be apparent fromthe description and the drawing, Y

in which the one figure is a fragmentary and diagrammatic representationof the form of this invention chosen for illustration, the digesterbeing shown in vertical crosssection.

Although the law requires a full and exact description of at least oneform of the invention, such as that which follows, it is,tof course, thepurpose of a patent to cover each new inventive concept therein nomatter how it may later be disguised by variations in form or additionsof further improvements; and the appended claims are intended toaccomplishV this purpose by particularly pointing out the parts,improvements, or combinations in which the inventive concepts are found.

General description The invention has been illustrated in connectionwith digester 11, which may be conventional except as here described.Digesters are commonly provided with gas storage covers 12, which arevertically movable. Gas formed within the digester is trapped withinsuch covers and may be drawn oit by a gas drawoff tting 13 positionedabove the water level. The water level in the digester is maintainedapproximately constant by an overilow device `14 vented to theatmosphere: through a pipe 16, The digested or stabilized sludge settlesto the bottorn of the tank, from which point it is drawn oit by adraw-oit' pipe 17.

According to the present invention and according to the co-pendingapplication above mentioned, the overilow device 14 draws its water froma clarier 18. This clarier comprises `a skirt 19 into which liquid canflow from the tank only through the bottom of the skirt. According tothe illustrated form of the invention, skirt 19 is closed at its top,thus forming a hood. It could, however, extend upwardly and be open tothe gas in cover 12. The closed hood is preferred at the present timefor reasons of economy and to make room for the apparatus locatedthereabove and described hereafter. Located within the skirt 19 and nearthe top thereof is a smoothow draw-off device 21. This is designed todraw approximately equally from widely-spaced points so as not to causeagitation or channeling within the skirt 19.

Although both solids and water will enter skirt 19 at the. bottomthereof, the rate. of upward liow within the skirt 19 is slow enough sothat the solids have time to settle and reasonably clear water is drawnthrough the draw-off device 21, through connecting pipe 22 to overflowdevice 14 and through effluent discharge pipe 23 to any desired point ofdisposal. A high degree of clarity for this effluent is not required,especially if the common practice is followed of returning it to thesewage-treating unit so that it passes through the final settling tank.The clarifier 19 therefore need not be exceptionally large so as toprovide a relatively slow rate of upward flow therethrough. A rate offlow of not over two gallons per minute per square foot has been foundto give satisfactory results.

The skirt 19 preferably extends to a low enough level in the tank sothat it is below the level of the circulating roll and below the levelat which most of the gas formation takes place. Hence there is no needto protect it from rising gases. This low level also facilities itsclarifying function, because only the relatively dense settled solidswhich are stabilized and hence usually free from clinging gas enter theskirt 19.

Positioned at a level substantially lower than the skirt 19 and henceconveniently positioned on top of the clarifier 18 is a gas-liberatingdevice 26. If it did nothing else, this gas liberator device would beuseful in causing a desirable circulation within the digester asdescribed. To this end it may conveniently comprise a vertical tube 27,into which gas is liberated through perforations by a jacket 28. Tube 27thus functions as an airlift pump to draw the digester liquor, includingsolids, into its open bottom and discharge them at its open top, theairlift action continuing thereabove.

According to the preferred form of the present invention, this gas whichproduces the circulation also heats the digester contents. Thus thejacket 28 is supplied with hot gases from a hot gas producer 29 which isconnected to jacket 28 through pipe 30.

The hot gas producer 29 may be of a known type, including a sealedcombustion chamber 31 fitted with burner 32 and at the other end withpipe 30, or a fitting 33 which may be used for various purposes such asan excess pressure relief valve, a drip trap, and conceivably sprayingthe hot gases with water if preliminary partial cooling of the gases isnecessary. The combustion chamber 31 is cooled by a water jacket topreserve its life. The burner 32 is supplied with air under pressure byan air pump 34 and with gas under pressure by gas pump 36, both beingdriven by a motor 37. The air pump 34 may draw the air from theatmosphere, preferably through a heat exchanger to heat the air by wasteheat for better efiiciency. The gas pump 36 preferably draws gas fromdraw-ofi fitting 13 through pipes 38 and convenient safety andgas-drying unit 39.

The hot gasv producer 29 may be located at any convenient point, such asin a nearby building. It is preferably above the level of the water inthe digester 11.

The hot products of combustion are thus liberated into the tube 27.Inasmuch as these gases are liberated through perforations and passupwardly as moderately small bubbles, the heat transfer between them andthe liquid is quite eicient and they give off most of their usable heatbefore they reach the surface, thus warming the contents of thedigester.

A hood 41 is desirable and is positioned with its lower edge below thewater level so that it is sealed from the liquid by the gas passingwithin the cover 12. This hood 41 is centered over the tube 27 so thatthe great majority of the gases which are liberated through the tube 27are caught by the hood 41. The hood 41 may communicate with a vent 42 sothat these spent gases may pass to the atmosphere. The hood 41,including the tube extending upwardly, should be large enough indiameter so thatliquid will not be carried up in i t excessively byairlift action.

An emergency overflow or draw-off valve 46 is preferably provided,connected as shown, and also a bleedoff valve 47. Opening thelatter maybe helpful if occasional charges of raw sludge are supplied to the tankin quantities which would cause excessive flow through clarifier 18. Thebleed valve will lower the level between the times of charging so thatexcessive hydrostatic pressure will not be produced by such charges.Both of valves 46 and 47 should be high enough not to carry the liquorlevel below the level required to seal the cover 12 in the latterslowest position.

The diameter of the casing or skirt 19 could be such that during pumpingof sludge into the tank a maximum rate of rise suitable for settling thereadily settleable solids will not be exceeded. In most instances a rateof rise of 2 U. S. gallons per minute per square foot is sufficientlyslow. The casing or skirt 19 need not be this large, however. Betweenpumpings, which usually do not last more than five minutes, the contentsof casing 19 have a long quiescent period. Hence it is only necessarythat the capacity of the casing below the draw-olf be equal to thevolume of sludge pumped in five or ten minutes, the latter giving amargin of safety. lt may be assumed that sludge will only be pumped fromone hopper, or class of hoppers (a group connected for simultaneouspumping) at a time.

The disclosure of application Serial No. 103,235 is incorporated hereinby reference.

I claim:

l. A digester for solids gathered in waste treatmentV plants including atank having an upper zone of substantially unrestricted circulation andin which gas formation occurs, an outliow conduit having means formaintaining the liquidV in the tank at a given level, a partition withinthe tank forming an isolated clarifier chamber constantly communicatingwith the tank only 'at a depth below the level of substantial gasformation to receive solids and liquid from the tank and to allow thesolids to settle back into the tank at any time, and a slow flowdraw-oftl intake in the clarifier chamber below the liquid levell butsubstantially above the level of communication between the chamber andthe tank for withdrawing clarified liquor from the chamber, said intakecommunicating with said conduit.

2. A digester for solids gathered in waste treatment plants including atank having an upper zone of substantially unrestricted circulation andin which gas formation occurs, an outflow conduit having means formaintaining the liquid in the tank at a given level, a partition withinthe tank forming an isolated clarifier chamber constantly communicatingwith the tank only at a depth below the level of substantial gasformation to receive solids and liquid from the tank and to allow thesolids to settle back into the tank, and a slow flow draw-off intake inthe clarifier chamber below the liquid level but substantially above thelevel of communication between the chamber and the tank for withdrawingclarified liquor from the chamber, said intake communicating with saidconduit, said partition comprising a tube open at its bottom. Y

3. A digester for solids gathered in waste treatment plants including atank having an upper zone of substantially unrestricted circulation andin which gas formation occurs, an outflow conduit having means formaintaining the liquid in the tank at a given level, a partition withinthe tank forming an isolated clarifier chamber constantly communicatingwith the tank only at a depth below the level of substantial gasformation to receive solids and liquid from the tank and to allow thesolids to settle back into the tank at any time, and a slow ow draw-offintake in the clarifier chamber below the liquid level but substantiallyabove the level of communication between the chamber and the tank forwithdrawing clarified liquor from the chamber, said intake communicatingwith said conduit, the diameter of said clarifier chamber beingSuthcient to provide a maximum rate of rise therein of not over twogallons per minute per square foot.

4. A digester for solids gathered in waste treatment plants including atank having an upper zone of substantially unrestricted circulation andin which gas formation occurs, raw sludge inilow means discharging intosaid tank, a partition within the tank forming a clarifier chamber andhaving a large opening communicating constantly with the tank only at adepth below the level of substantial gas formation to receive solids andliquid from the tank and to allow the solids to settle back into the vtank at any time, said inflow means communicating with the tank onlyoutside of the partition; and means for drawing off liquor from onlywithin the clariier chamber, including means for maintaining the liquidin the tank at a given level, and a draw-01T intake in the upper thirdof the clarier chamber below the liquid level but substantially abovethe level of communication between the chamber and the tank forwithdrawing clarified liquor from the chamber.

5. A digester according to the preceding claim, having means for causingcirculation of the sludge at levels above the level of the communicationbetween the chamber and the tank.

6. The method of withdrawing supernatant liquor from a digester havingan upper zone of substantially unrestricted circulation and in which gasformation occurs which comprises liowing liquid and solids from onlybelow the level of substantial gas formation into the lower part of aclarifier having no other supply and no other communication with theliquid content of the tank, passing the liquid up through the clarifierwith a rate of rise not over two gallons per minute per square foot,allowing the solids to settle back into the digester at any time andwithdrawing clarified supernatant liquor from an upper level of theclarifier.

7. The method of withdrawing supernantant liquor from a digester havingan upper zone of substantially unrestricted circulation and in which gasformation occurs which comprises owing liquid and solids from only alower well-digested sludge zone into only the lower part of a clarifierhaving no other supply and no other communication with the liquidcontent of the tank, passing the liquid up through the clarifier,retaining it in the clarier long enough to permit the readily settleablesolids to settle therefrom, and withdrawing clarified supernatant liquorfrom an upper level of the clarier.

References Cited in the tile of this patent UNITED STATES PATENTS1,820,976 Imhoft Sept. 1, 1931 1,987,888 Babbitt et al Jan. 15, 19352,188,847 Streander Jan. 20, 1940 2,263,451 Bach Nov. 18, 1941 2,336,659Welp Dec. 14, 1943 2,373,154 Welp Apr. 10, 1945 2,409,585 Pratt Oct. 15,1946 2,538,412 Cecil et al. Ian. 16, 1951 2,638,444 Kappe May 12, 1953FOREIGN PATENTS 441,851 Germany Mar. 16, 1927

1. A DIGESTER FOR SOLIDS GATHERED IN WASTE TREATMENT PLANTS INCLUDING ATANK HAVING AN UPPER ZONE OF SUBSTANTIALLY UNRESTRICTED CIRCULATION ANDIN WHICH GAS FORMATION OCCURS, AN OUTFLOW CONDUIT HAVING MEANS FORMAINTAINING THE LIQUID IN THE TANK AT A GIVEN LEVEL, A PARTITION WITHINTHE TANK FORMING AN ISOLATED CLARIFIER CHAMBER CONSTANTLY COMMUNICATINGWITH THE TANK ONLY AT A DEPTH BELOW THE LEVEL OF SUBSTANTIAL GASFORMATION TO RECEIVE SOLIDS AND LIQUID FROM THE TANK AND TO ALLOW THESOLIDS TO SETTLE BACK INTO THE TANK AT ANY TIME, AND A SLOW FLOWDRAW-OFF INTAKE IN THE CLARIFIER CHAMBER BELOW THE LIQUID LEVEL BUTSUBSTANTIALLY ABOVE THE LEVEL OF COMMUNICATION BETWEEN THE CHAMBER ANDTHE TANK FOR WITHDRAWING CLARIFIED LIQUOR FROM THE CHAMBER, SAID INTAKECOMMUNICATING WITH SAID CONDUIT.